In an exhaust gas recirculation apparatus of the so-called exhaust gas pressure control type, the pressure of the exhaust gas in the exhaust gas recirculation passageway connecting the exhaust manifold to the intake manifold can be maintained at a predetermined constant value for every type of engine load condition. Therefore, the ratio of the amount of the inert exhaust gas introduced into the intake manifold to the amount of total gas (including air-fuel mixture and the recirculated exhaust gas) introduced into the engine combustion chamber, which is called the external EGR ratio, is controlled to a predetermined constant value for every type of engine load condition, as is well known to those who are skilled in this art.
However, an ideal exhaust gas recirculation operation cannot be performed only by controlling the external EGR ratio to said predetermined value, due to the existence of the inert combustion gas which remains in the combustion chambers of the engine without being exhausted. In this case, the ratio of the amount of the inert combustion gas remaining in the combustion chambers, to the amount of the total gas introduced into the combustion chambers, which is called the internal EGR ratio, is decreased in accordance with an increase in the load of the engine. Due to this fact, the so-called required EGR ratio cannot be controlled to a predetermined value, which is a ratio of the amount of the inert gas introduced into the intake manifold from the exhaust manifold and remaining in the combustion chambers, to the amount of the total gas introduced into the combustion chambers.
To maintain the required EGR ratio to a predetermined constant value an exhaust gas recirculation apparatus has heretofore been provided, wherein a flow control valve is arranged on the exhaust gas recirculation passageway to control the amount of exhaust gas in such a manner that the external EGR ratio is increased in accordance with an increase in the load of the engine. The required EGR ratio can therefore be controlled to a predetermined constant value. The flow control valve has a spring-urged diaphragm connected to a valve member facing a valve seat. To a vacuum chamber formed on one side of the spring-urged diaphragm, a vacuum signal corresponding to the engine load is transmitted to control the distance between the valve member and the valve seat for controlling the amount of exhaust gas, which corresponds to the external EGR ratio, in accordance with the engine load.
However, this type of exhaust gas recirculation apparatus exhibits drawbacks wherein the required EGR ratio is apt to be decreased during low load conditions due to the use of a single-flow control mechanism comprised of the spring-urged diaphragm, the valve member and the valve seat, and wherein parts with high accuracy are necessary for obtaining a required constant EGR ratio during every type of load conditions.